I don't commute any more, but for many years i did so over various distances. The longest, for one year, was 29 miles each way. That was a substantial slice out of each day, so I did that only three or four days per week. The commute I did for the longest time was 16 miles each way, and I came to regard this as pretty much an ideal distance - it took just about an hour, and two hours a day five days a week was both enjoyable and gave me a decent base level of fitness on which to build.